Raman spectroscopy, 19F and 31P MAS-NMR of a series of fluorochloroapatites

Abstract

We report the Raman spectra of a series of fluorochloroapatites Ca 5(PO 4) 3F 1− x Cl x , where x = 0.0, 0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 0.9 and 1.0 (i.e. from pure fluorapatite to chlorapatite). The series did not appear to exhibit any immiscibility, however peak broadening and additional Raman and NMR spectral features on chlorine addition could be interpreted as reduction in symmetry and ordering in the ion channels. All Raman bands showed significant broadening with chlorine substitution, indicating disordering of the crystal lattice or ordering into fluorine and chlorine rich environments. There was also a general trend of the Raman bands to shift to lower wavenumber linearly with chlorine substitution with the ν 1 phosphate band decreasing from 966 cm −1 for x = 0.0 to 961 cm −1 for x = 1.0. The full width half maximum (FWHM) of this band increases linearly with chlorine addition from 5 cm −1 for x = 0.0 to 10 cm −1 for x = 1.0. The shift in a component in the ν 3 phosphate band with chlorine addition at around 1035 cm −1 agrees with data in the literature on chlorine containing geological apatites. An additional component was seen at around 586 cm −1 in the ν 4 phosphate region for chlorapatite and the area of this band decreased linearly to zero as fluorine replaced chlorine. This could be due to E g symmetry phonons splitting into A g and B g modes due to a symmetry change such as hexagonal to monoclinic with chlorine addition. 19F magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra showed a shift to lower ppm values with chlorine addition and a broadening of resonances, consistent with the Raman data. The 31P spectra developed additional shoulders (at around 2 and 4 ppm) with chlorine addition with the main peak position (3.3 ppm for x = 0) decreasing for compositions moving away from x = 0.5 indicating maximum phosphorous nuclear shielding occurs at an approximate F:Cl ratio of 1:1. This discontinuity is a possible indication of a structural transition at x = 0.5 related to local short scale phosphate order ↔ disorder or a change in crystal symmetry.